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Particle model theory applied to energy transfer in state changes
IGCSE AQA GCSE Physics Edexcel GCSE Physics OCR GCSE
Gateway Science Physics OCR GCSE 21st Century Science Physics
Doc Brown's school physics revision notes: GCSE
physics, IGCSE physics, O level physics, ~US grades 8, 9 and 10
school science courses or equivalent for ~14-16 year old students of
physics
What is internal
energy and latent heat?
Particle motion in gases and gas pressure
Sub-index for this page
1.
Introduction
to using kinetic
particle theory to explain the states of matter
2.
What is the internal energy of a substance
3.
Energy transfer in state changes and conservation of mass
4a.
Introduction to latent heat and state
changes
4b.
A heating Curve
- steadily increasing the internal energy of a system
4c.
A cooling Curve
- steadily decreasing the internal energy of a system
4d.
Some everyday examples of latent
heat - internal energy transfers
4e.
Defining specific latent heat
4f.
Examples of worked-out
heat calculations involving specific latent heat
5a.
The particle model of a gas - motion and gas pressure
5b.
Considering the
internal and external pressures of a container of gas - the effects of
changing quantity, volume or temperature
5c. Increasing the energy
store of gas - work done and temperature effects
6.
Factors that affect
the rate of evaporation and condensation
7.
What is the lowest temperature possible? Kelvin absolute
temperature scale
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1.
INTRODUCTION
to using kinetic
particle theory explaining properties of three different states of matter
The particle model has been developed to explain the
properties of the three states of matter, namely gas, liquid and solid.
The particle model also provides a way of describing the
changes of state between a gas, liquid and the solid state of a material.
To change the state of a material requires either the
input of heat or the removal of heat from the material and this is called the
latent heat. and consider
the concept of internal energy.
The 'model' particle pictures below give you an idea of how the
states of matter (gas, liquid and solid) are viewed when applying the theoretical ideas
to explain how the three states of matter behave, especially when subjected to a
change in temperature.
You should be able to recognise
simple diagrams to model the difference between solids, liquids and gases
- the three states of matter.
Gases: There are almost
no forces of attraction between gas particles, they have the most kinetic
energy of the three states, the particles are completely free to move around
at random and they move at high speeds in all directions - so they have a
higher kinetic energy store than liquids. The free moving
particles have kinetic energy of movement and there is much empty space
between the particles.
Liquids: There are weak
forces of attraction between liquid particles (if there wasn't, you couldn't
have a liquid!), the particles are relatively close together but free to
move around at random but with lower speeds than in the gas. The free moving
particles still have kinetic energy of movement from one place to another - not quite as high a
kinetic energy store as gases.
Solids: In solids there
are stronger forces of attraction between the particles which prevents the
particles moving around and passing each other. The particles are held in
fixed positions in a regular arrangement. Their even lower kinetic energy is
due to the particles (atoms or molecules) vibrating around their mean or
average positions in the crystal structure. So solids have virtually no movement kinetic energy store
from one place to another, as in the case of gases or liquids.
See also
The density of materials and the particle model of matter
and
More detailed descriptions of states
of matter
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2. What is the internal energy of a substance
(KE shorthand for kinetic energy)
-
The particles of solids,
liquids and gases all have kinetic energy (KE).
-
In solids the particles vibrate with
kinetic energy but can't move around to another position, but in gases and
liquids the particles freely move from place to place with kinetic energy.
-
Particles also have
energy in their potential energy stores due to their positions -
the motion from their kinetic energy keeps them separated as it opposes the
forces attracting the particles together.
-
The particles in gases have the most
potential energy because they are the farthest apart.
-
In potential energy order: gases >>
liquids > solids
-
Remember there is a little space on
average between liquid particles, but virtually non between the particles of
a solid.
-
Therefore the internal energy of a system is stored
by the particles (atoms, ions, molecules) because of their kinetic energy
and spacing-position.
-
When you heat a system energy is
transferred to the particles eg they move faster in gases and liquids
(increase in KE of movement from one place to another) or the particles
vibrate more strongly in a solid (increase in vibrational KE), so the
internal energy is increased when you heat a material.
-
This absorption of heat, ie increase in
internal energy can cause an increase in temperature OR a change of state
e.g. melting or boiling if the particles are given sufficient thermal
energy.
-
Removing heat decreases the internal
energy, so the material cools to a lower temperature OR undergoes a change
of state e.g. condensing or freezing.
-
The size of the change depends on the
energy input, the mass of substance involved and the specific
heat capacity (which depends on the nature of the material).
-
See
Specific heat capacity: how to determine it, use of data,
calculations and thermal energy stores
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3.
Energy transfer in state changes and conservation of mass
 |
FREEZING
MELTING
 |
 |
|
SUBLIMING |
 |
|
 |
BOILING or EVAPORATING |
|
SUMMARY of the CHANGES of STATE between a gas, liquid and solid
All mass conserved in these
PHYSICAL CHANGES |
 |
 |
CONDENSING
These are NOT
chemical changes ! |
-
As well as the transfer of heat energy by
conduction, convection and radiation, state changes like evaporation and condensation
also involves heat energy transfers and the particle model can be used to
explain them.
-
ON HEATING
- adding thermal kinetic energy, increasing internal energy
-
When you heat a solid, the
vibrational kinetic energy of the particles is increased until they have enough KE to
weaken the interparticle bonds to allow melting and the particles are free
to move around in the liquid state.
-
With further heating above
the melting point, the particles gain more kinetic energy and the inter-particle bonds are further weakened so that the particles at the surface with the highest KE can
escape the surface (evaporate) or vapourise to the gaseous state in the bulk liquid
(bubbles!) at the boiling point.
-
The graph below shows how the distribution of kinetic energy and speed of
particles changes with changes in temperature - with increase in
temperature, the average speed and kinetic energy of the particles increases.
-
Note that the random movement and collisions of the
particles creates a wide range of speeds/kinetic energies.
-
-
When the temperature is increased, more particles have a
greater kinetic energy and greater speed, but only the highest
speed/kinetic energy particles can escape from the surface (only the
very right-hand section of the graph curves)
-
Below is a
particle model of evaporation.
-
-
ON COOLING
- removing thermal kinetic energy - decreasing internal energy
-
If you cool the substance, the reverse
happens e.g. cool a gas so the interparticle bonds bring the particles
together to condense and form a liquid.
-
Further cooling reduces the KE of the
liquid particles so that when the temperature is reduced to the freezing
point, the interparticle forces are sufficient to 'club' the particles
together to form a solid.
-
All these physical state changes are
reversible by adding or removing thermal energy, no new substances are formed
(NOT a chemical change) and all
mass is conserved. What you start with is
what you finish with and all the original properties are retained.
-
The only difference between the states
of a substance is how the particles are arranged (as described in
section 1. above).
-
Note that in
a closed system, mass is
conserved in a system undergoing a change in state.
-
If you melt 100 g of ice, you get 100 g
of water!
-
However, even with mass conservation, you
can get a volume change, except for water, for the same mass, liquids occupy
a slightly larger volume and gases occupy a massively greater volume than
the liquid or solid form.
-
Ice is unusual that the solid ice
crystals are less dense than water - which is why ice floats!
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4.
State changes and latent heat
4a.
Introduction to
latent heat and changes of state g <=> l <=> s
-
The energy needed to change the physical sate of
substance at constant temperature is called the latent heat.
-
There are two latent heat values:
-
A historical curiosity - latent heat
('hidden' heat), which was unexplained until the particle theory of matter
was developed and inter-particle bonding understood.
-
Changes of state can be represented as a
temperature - time graphs.
-
Heating curve - increasing temperature
due to the addition of thermal energy, increasing
the internal energy of the system.
-
Cooling curve - decreasing temperature
due to the removal of thermal energy, decreasing the
internal energy of the system.
-
BUT, the graphs are not simple 'curves',
there are horizontal sections that need explaining using the concept of
latent heat.
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4b. A heating Curve
- steadily increasing the internal energy of a system
-
When a solid is heated from the solid
state to the gaseous state and the temperature of the system measured
continuously, there are two horizontal sections on the graph where the
temperature does not rise, despite the constant input of thermal energy
(continuous heating). Typical results are shown in the heating curve
graph below.
-
 This
is called a HEATING CURVE
-
As you heat the substance you are increasing the internal energy. BUT the temperature stays constant during the state changes of melting
at temperature Tm and boiling at temperature Tb (see diagram above).
-
This is because all the extra ('hidden') energy absorbed in
heating at these two temperatures
(called the latent heat of state change),
goes into weakening the inter–particle
forces (intermolecular bonds) .
-
The thermal energy gain at this point equals the
heat energy
absorbed needed to
reduce the interparticle forces in melting or boiling - the latent heat.
-
During the state change the temperature stays
constant until all the latent heat is absorbed and the state change
completed, so no temperature rise can occur.
-
In between the 'horizontal' state change
sections of the graph, you can see the energy input increases the
kinetic energy of the particles and raising the temperature of the
substance as you expect as the internal energy increases.
-
For these state changes you have the addition of the
latent heat of melting at temperature Tm and the addition of the
latent heat of boiling at temperature Tb.
-
The diagram involving the
brown half-arrows
illustrates what is happening to the energy stores in a heating curve.
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4c.
A cooling Curve
- steadily decreasing the internal energy of a system
-
As you cool a substance you are decreasing the internal energy. BUT the temperature stays constant during the state changes of
condensing
at temperature Tc and freezing at temperature Tf (see diagram below).
-
Similarly when a gas is cooled from
the gaseous state to the solid state and the temperature of the system
measured continuously, there are two horizontal sections on the graph
where the temperature does not fall, despite the constant removal of
heat energy (continuous cooling). Typical results are shown in the cooling curve graph below.
-
 This
is called a COOLING CURVE
-
As you cool the substance you are decreasing the internal energy. BUT the temperature stays constant during the state changes of condensing
at temperature Tc, and freezing/solidifying at temperature Tf.
-
This is because all the extra
('hidden') heat energy removed on cooling at these temperatures (the
latent heat of state change), reduces the
KE and potential energy of the particles.
-
The heat loss is compensated by the increased intermolecular force
attraction which releases heat energy.
-
During the state change the temperature stays
constant until all the latent heat is removed and the state change
completed, so no temperature fall can occur.
-
In between the 'horizontal' state change sections of the
graph, you can see the energy 'removal' reduces the kinetic energy of
the particles, lowering the temperature of the substance.
-
For these state changes you have the removal of the
latent heat of condensation at temperature Tc and the removal of
the latent heat of freezing at temperature Tf.
-
The diagram involving the
blue half-arrows
illustrates what is happening to the energy stores in a cooling curve.
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4d. Some everyday examples of latent
heat - internal energy transfers
-
Whenever materials at different
temperatures are placed in contact with each other, there will be an
internal energy transfer of thermal energy from the hotter material to the
colder material.
-
(1)
Using ice to cool a drink
-
When you add ice to a drink to cool it,
an internal energy change takes place involving the latent heat of
melting.
-
The ice is at a lower temperature than
the liquid drink.
-
The higher energy liquid particles
transfer kinetic energy to the ice, increasing its internal energy.
-
Sufficient thermal energy - the latent
heat of melting, is absorbed by the ice to melt it.
-
The energy is needed to weaken the
intermolecular forces between the water molecules in the ice
sufficiently to cause melting - when the particles have sufficient energy to
break free from the inter-particle attractive forces.
-
The ice warms up and your drink cools
down - an internal thermal energy transfer!
-
(2)
Refrigerator -
freezer
-
In a refrigerator system, an electric
pump is used to compress a gas to liquefy it - this pump-compressor
is forcing condensation to take place and releases the latent heat of
condensation.
-
The liquid is then allowed to evaporate,
absorbing its latent heat of vapourisation.
-
This thermal internal energy is taken
from the contents of the fridge-freezer.
-
The internal energy of the freezer
contents is reduced, lowering the temperature of the food.
-
You can feel the warm air at the back of
your fridge, this is from the release of the latent heat of condensation.
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4e. Defining specific latent heat
-
The specific latent heat of a
substance is the quantity of energy need to change 1 kg of the material
from one state to another without change in temperature.
-
(a) In heating a material to effect a
state change e.g. melting or boiling, the specific latent heat must be
added.
-
(b) In cooling a material to effect a
state change e.g. condensing or freezing, the specific latent heat must
be removed (released) from the system.
-
Specific latent heat values differ
from substance to substance because of different values of
inter-particle forces (intermolecular bonding) and also the state change
itself for a specific substance (solid <=> liquid OR liquid<=> gas).
-
Generally speaking latent heats of
boiling/condensing are numerical much greater than latent heats of
melting/freezing.
-
The latent heat for the state changes
solid <=> liquid is called the specific latent heat of fusion
(for melting or freezing).
-
The latent heat for the state changes
liquid <=> gas is called the specific latent heat of
vaporisation (for condensing, evaporation or boiling)
-
Specific
heat capacity is dealt with on a separate page
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4f.
Examples of worked-out specific latent heat
calculations
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5a.
The particle model of a gas - motion and gas pressure
-
All
particles have mass and their movement gives them kinetic energy and
momentum.
-
The particles in a gas are in constant
random motion - random direction, variety of velocities and kinetic
energies.
-
Although the collisions occur at random
in any direction, there is a resultant force acting at right angles to
any surface.
-
There will always be a gas pressure,
unless a container is under vacuum, no particles - no collisions - no
pressure!
-
When the fast moving gas particles
collide with a surface, their millions of impacts create a force that we
measure as gas pressure - the total force of impacts per unit area.
-
The particles collide with the container
surface completely at random and impact at every angle, BUT, the effect is
to create a net force at right angles to the surface - gas pressure!
-
The more forceful the collisions on a
surface or the greater the number of collisions per unit area of surface,
the greater the pressure, assuming the gas volume keeps constant.
-
If the temperature is kept constant and
the volume increased, the impacts are more spread out and less frequent per
unit area, so the gas pressure decreases.
-
Conversely, if a gas is
compressed into a smaller volume at constant temperature, the number
of impacts per unit area increases, so the pressure increases.
-
If the sides of a gas container
are 'flexible' (e.g. balloon), the volume will only be constant when
the internal and external pressures are equal.
-
 From measurements of volumes and
pressure of gases at constant pressure, a numerical inverse law can
be formulated - see graph on right.
-
pressure x volume = a constant
(at constant temperature)
-
pV = constant
-
p = pressure in pascals (Pa =
N/m2),
V = volume (m3)
-
You can connect two pressure and
two volumes by the simple equation
-
p1 x V1
= p2 x V2
-
where 1 represent the original
conditions, and 2 the final situation if an enforced change of p1
or V1 is made.
-
Examples of simple gas
calculations
For more gas calculation see
P-V-T pressure-volume-temperature gas
laws and calculations
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5b. Considering the
internal and external pressures of a container of gas
Pressure in fluids
Fluids are materials that can flow
because the attractive forces between the particles are weak in liquids
and almost non-existent in gases.
Since the particles are free to move,
they collide with any surface they make contact with.
This produces a net resultant force
at 90o to the surface.
The basic formula for pressure is:
Pressure = Force normal to the
surface ÷ area of contact surface
P (Pa) = F (N)
÷ A (m2)
For more on liquid
fluid and atmospheric pressure see:
Pressure in liquid fluids and hydraulic
systems
Pressure & upthrust in liquids, why do
objects float/sink?, variation of atmospheric pressure with
height
However, here, I'm only concerned
with explaining more about gas pressure, using the model illustrated below
to describe, explain and quantify the behaviour of a gas.
The effects of changing
the amount or
temperature of a gas in a container
The particles in a gas are in constant
motion - flying around in all directions with frequent collisions (e.g.
in air the collision rate is 109/s !!!).
As already described, increasing the
temperature of a gas, increases the kinetic energy store of the gas
particles.
This is the kinetic energy of
movement from one place to another, its not vibrational kinetic energy.
In fact, the average kinetic energy of
the gas particles is directly related to the temperature.
The higher the gas temperature, the
greater the average kinetic energy of the particles,
and the cooler the gas the lower the
average kinetic energy of the particles.
As you increase temperature, the
average speed of the particles increases and the average kinetic
energy - remember the kinetic energy formula:
KE = ½mv2
(m = mass of particle, v = velocity of particle)
We can now discuss particular 'pressure'
situations and the starting point is the fact that ...
... gas pressure is caused by the
collision of particles with any surface ...
... because when particles collide
with a surface, the exert a force on that surface.
Pressure is related to the number or
force of particle impacts per unit area of the surface.
The more impacts or more forceful
impacts on the surface, the greater the pressure created.
Increasing temperature of a gas
actually increases both.
-
-
(i) Consider a steel cylinder of gas -
a rigid containing wall
-
When a gas is contained a rigid vessel
you can pump lots of gas in to a pressure much higher than the surrounding
atmospheric pressure.
-
Steel cylinders are used in industry to
store gaseous chemicals and in the home we used cylinders of hydrocarbon
gases for heating and cooking.
-
The effect of increasing
the amount of gas in the cylinder
-
The more gas you force in, the greater
the internal pressure because of the increase in the number of particle impacts per
unit area - a greater concentration of particles means more impacts on
the same surface area.
-
For a given cylinder, the gas volume is
constant and the pressure is proportional to the amount of gas pumped in at
constant temperature.
-
Pressure and volume are inversely
proportional to each other.
-
P x V = constant, P =
pressure in Pa (pascals), V = volume in m3.
-
At constant temperature, increasing the
volume decreases pressure because the collisions are more spread out over
the same area - less particle collisions per unit area.
-
At constant temperature, decreasing the
volume increases pressure because the collisions are more concentrated over
the same area - more particle collisions per unit area.
-
See also
P-V-T pressure-volume-temperature gas
laws and calculations
-
If the internal and external pressures are
not balanced, that's no problem with a strong steel walled cylinder!
-
The effect of increasing the
temperature of the gas in a cylinder
-
If the cylinder is heated it will expand
a little, but this will not compensate for the increase in gas pressure as
the gas tries to expand.
-
If the cylinder and its contents increase
in temperature, then the thermal energy store is increased as the gas
particles gain kinetic energy.
-
This increase in the particle kinetic
energy store increases the rate of particle collision AND the force of the
particle impacts on the container surface - thus raising the pressure with
increase in temperature.
-
This is quite a dangerous situation that
fire-fighters face when tackling a fire at a factory where gas cylinders are
used - the high temperatures and high pressures created in the gas cylinders
will cause them to explode violently.
-
-
(ii) Consider a balloon of gas - a
flexible containing wall
-
If the sides of a gas
container are 'flexible' (e.g. like a balloon), the volume will only
be constant when the internal and external pressures are equal.
-
If the external pressure is greater than
the internal pressure the balloon will decrease in volume (size).
-
If the internal pressure is greater than
the external pressure the balloon will increase in volume (inflate).
-
To blow up the balloon you blow in
with a force greater than atmospheric pressure to create the volume of
trapped gas.
-
The size of the balloon is then determined by how much air
you have blown in and the ambient atmospheric pressure.
-
The pressure of a gas in a balloon produces a net outward force at right angles to the
container surface due to the internal gas particle impacts.
-
BUT, as you observe with a blown-up
balloon, it doesn't seem to be expanding or contracting.
-
The reason being that the external air
particle impacts on the outside surface of the balloon create an opposing
and equal balancing pressure.
-
By blowing in air you increase the
internal pressure and force the balloon to expand, pushing the rubber skin
outwards, until the internal and external pressures are equal when expansion
will stop.
-
When you blow in you are increasing the
number of particle impacts per unit area of the internal surface to create
the greater outward acting force.
-
Remember, increasing the volume of a gas
at constant temperature decreases the pressure (pV = constant).
-
The pressure you create initially when
blowing up the balloon, must decrease as it expands - less particle impacts
per unit area.
-
If you let air out of the balloon, or it
leaks out, there are less particle impacts per unit area of surface and the
pressure is reduced, so the greater external pressure causes the balloon to
contract until the volume is reduced creating a pressure equal to the
external atmospheric pressure.
-
If a balloon inflated with air is
heated, the gas particles inside will increase in kinetic energy
producing more collisions and more forceful collisions - increase in net
force acting on the surface.
-
Therefore the pressure increases and the
balloon expands.
-
BUT, the expansion spreads out the
collisions (which decreases pressure - less force per unit area), so the
balloon only expands until the internal pressure equals the external
pressure of the cooler air.
-
When the balloon cools down it will
decrease in size, less forceful particle collisions, balloon shrinks until,
again, the internal and external pressures are equal.
-
When helium weather balloons are
released, they rapidly rise up through the atmosphere and greatly expand
because atmospheric pressure significantly decreases with increase in
height above the earth's surface.
-
As the external pressure decreases
(less particle impacts per unit area) the internal pressure is greater (more
impacts) and so the greater number of internal impacts per unit area force the volume of
the gas in the balloon to increase.
-
The helium balloon will continue to
expand as long as the external pressure is less than the internal pressure.
-
It will stop expanding when the internal
balloon pressure drops to the same as the external pressure.
-
However, since it is filled with less
dense helium, it will continue to rise and rise!
-
(iii) The same arguments apply to blowing
up a bicycle tyre or motor car tyre or anything else!
-
Any increases in the external
pressure from a pump system will allow expansion of the tyre if it
exceeds the internal pressure inside the tyre - otherwise no further
inflation!
-
When you seal the end of a gas
syringe (like you see in chemistry), with your hand and press the
plunger in.
-
You can compress the air to create a greater gas pressure
than the external atmospheric pressure. BUT, although the pressures are
not initially balanced, as in the case of blowing up balloon, its your extra
muscle force that helps create the balancing force.
-
internal pressure in syringe =
atmospheric pressure + pressure from muscle force
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5c. Increasing the energy store of gas - work done and
temperature effects
-
Increasing the energy store of gas
by compressing it
-
When you pump air into a bicycle tyre
as energetically as you can, you can detect a rise in temperature,
particularly near the pump connection point. So, why the increase in
temperature of the gas?
-
When you compress a gas by applying a
mechanical force you do work of compression on the gas.
-
This work in compressing the gas
increases the internal energy and increases the temperature - increasing
the thermal energy store - the kinetic energy store is increased.
-
You have to do work on the gas
because as you compress the air in the pump, the pressure rises as the
force of the particle impacts acts against you, so you
have to do work against this increased force/unit area (pressure) to get the air into
the tyre.
-
By doing work on a gas in this way
the increase in the internal energy store of the gas ends up as
increased kinetic energy of the particles, which causes the temperature
rise of the
air, tyre and pump.
-
This effect is used in refrigerators
where a refrigerant gas is compressed to release energy in a closed
system - this thermal energy is obtained from the refrigerant liquid
evaporating by absorbing the latent heat of vaporisation from the
interior of the fridge-freezer.
-
If you compress a gas, decreasing
its volume, you increase its internal energy, increasing the average
kinetic energy of the particles and the gas gets warmer with an increase
its temperature.
-
If you expand a gas, increasing
its volume, you decrease its internal energy, decreasing the average
kinetic energy of the particles, the gas cools as the temperature is
reduced.
-
Increasing the energy store of gas
by heating it
-
Increasing the temperature of a gas
increases its kinetic energy store.
-
Increasing the temperature increases the
average speed of particles and their kinetic energy.
-
In fact, the temperature of a gas is
proportional to the average kinetic energy of the particles.
-
This means on heating a gas in a sealed
container there are more particle impacts and more forceful impacts on the
surface per unit area.
-
Therefore heating a gas at constant
volume increases the gas pressure.
-
Conversely if you cool and sealed
cylinder of gas, the pressure decreases.
-
More on
gases and more on
gas calculations
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6. Factors that affect
the rate of evaporation and condensation
-
Condensation occurs when
a gas/vapour is cooled sufficiently to a low enough temperature to allow the
attractive forces to be strong enough to attract the particles together as a
liquid. This can only happen if the kinetic energy of the particles is low
enough (the lower the temperature the smaller the kinetic energy).
-
Water vapour in the air
condenses out on cold surfaces in the winter eg window condensation,
invisible steam from a boiling kettle condenses out into clouds of tiny
droplets of water, which technically isn't steam! and rain drops form in the
higher cooler regions of the atmosphere.
-
Factors affecting the rate of
condensation
-
The cooler the gas, the faster
it condenses - more lower KE particles can be attracted together.
-
The lower the temperature of the
surface the gas is in contact with.
-
The lower the airflow over the
surface, this keeps the concentration of the condensing gas as high as
possible.
-
When a vapour/gas is condensed the
latent heat of vaporisation must be removed to cool the particles down
sufficiently for condensation to take place.
-
Because of this, being scalded by
steam is worse than be scalded by boiling hot water.
-
Both involve transfer of thermal
energy due to the heat capacity of liquid water.
-
BUT, water vapour must be first
condensed, so initially you are scalded by the release of the latent
heat of vaporisation = the 'latent heat of condensation'.
-
Evaporation is when the
highest kinetic energy particles of a liquid escape from the surface ie can
overcome the attractive forces of the bulk of particles. The greater the KE
of a liquid surface particle, the greater the chance to escape and become a
gas particle. Evaporation can take place at any temperature between a
substance's melting point and boiling point. As the highest KE particles
escape, leaving the slower lower KE particles, the bulk of the liquid will
cool, so a cooling effect accompanies the evaporation of a liquid. The
cooling effect of sweating is due to evaporation of water from your skin.
-
Factors affecting the rate of
evaporation
-
-
The higher the liquid
temperature, the faster the rate of evaporation - more particles with enough
kinetic energy to escape from the surface (graph above).
-
Reminder of particle model of evaporation
-
The greater the surface area,
the faster the evaporation - more area, more chance of evaporation.
-
The greater the airflow over the
surface of the faster the evaporation rate - the air can become saturated
with the vapour of the liquid, so it is more readily replaced if the already
evaporated liquid is swept away by air flowing over the surface.
-
When water evaporates the latent heat
of vaporisation is absorbed by the water molecules giving a cooling
effect.
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7.
What is the lowest temperature possible? Kelvin absolute temperature scale
We know the temperatures of the cores of
stars can be millions of degrees, so there doesn't seem to be any upper
limit to temperature!
BUT, is there limit at the lower end of
the temperature scale? The answer is YES!
 
The first experimental evidence for a
lower temperature limit came from graphs of volume versus temperature and
pressure versus temperature for a fixed mass of gas.
The plots for these were linear until the
gas liquefied or the liquid solidified.
BUT, if you extrapolated the gaseous data
back, you find every line ended up at a theoretical pressure of zero and
temperature of -273oC.
Explanation ....
As you cool a material, the particles
have less and less kinetic energy of movement around (gases or liquids) or
vibration in fixed positions (solid).
The kinetic energy of particles is a
function of temperature.
You can also say, that what we
measure as temperature, is a measure of the average kinetic energy
particles have.
BUT, eventually virtually all movement ceases
at a temperature of -273oC, particles have ~zero kinetic
energy.
Therefore that's it as regards
particle KE, and the temperature as we know it, cannot fall any further
- there is no more internal energy to remove!
So, the lowest possible temperature
that is -273oC. (-273 on the Celsius scale, unit
oC).
Theoretically, at this temperature, the
particles have no kinetic energy of movement or vibration, the coldest they
can be - nothing left in the kinetic energy store of the particles.
In fact by then, every substance will
have solidified, but at -273oC there is zero vibration of the
particles.
In 1848 a Scottish-Irish scientist called
William Thompson (later became Lord Kelvin) proposed a new temperature scale
starting at zero (called absolute zero), which became known as the
'Kelvin' scale of temperature - unit K.
Therefore the difference between the
Celsius and Kelvin scales is 273.
To convert from one to the other, the
following simple formulae apply.
K = 273 + oC
or oC = K - 273
(absolute zero 0 K is the same temperature as - 273oC)
Examples of temperature Celsius (oC) and
Kelvin (K) scale conversions:
Freezing point of water = 0oC,
therefore 0 + 273 = 273 K.
Boiling point of water = 100oC,
therefore 100 + 273 = 373 K.
Melting point of pure iron = 1811
K, therefore 1811 - 273 = 1538 oC
Note: Do NOT write degrees Kelvin, do
NOT write oK !!!,
and don't write just C for
Celsius, you need the degree symbol o too!!
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